California Common Sense reviewed data from
2000 through May, and it billed the report as the most
comprehensive study of its kind. Research director Mike Polyakov said it is "troubling" how difficult it is to track campaign
spending with the secretary of state’s online database,
known as Cal-Access.

“Correcting
for its inaccuracies and the poor organization of its interface, we sought to
provide this useful information to all Californians,” he said in a statement.

Independent expenditures are different from campaign donations. Since voters limited direct contributions in 2000 with Proposition 34, big donors and other special interests have increasingly used separate committees to spend money on behalf of candidates.

"Independent spending -– unhampered by limits -– can potentially be more influential than direct contributions," said the California Common Sense report.

Unions
and their campaign committees were the biggest players, accounting for $90 million of independent spending in the last decade, the report said. Businesses and related groups gave $27.7 million.